The present invention relates to the molding of tires, and in particular, radial tires for passenger cars or so-called "heavy vehicles", such as trucks or buses. It also concerns the molding of treads used for recapping, and in particular, it concerns the sculpture of the treads of such tires.
In order to effect the molding and vulcanizing of these tires, it is well known to use molds comprising two side parts which mold the side walls and an annular ring of segments arranged between the side parts to mold of the tread. By way of example, U.S. Pat. Nos. 3,847,520 and 3,779,677 describe such molds.
Based on the finding that certain tires for heavy vehicles, particularly when used on buses, caused vibrations and noise in frequencies ranging from 30 Hz to 150 Hz, it has been possible to establish that the cause of these vibrations is to be imputed to the molding of the tires.
The object of the present invention is, therefore, to provide a new concept for the construction of the molds in order to do away with the defects of lack of geometrical uniformity, irregularity and excess thickness which cause annoyance from the standpoint of comfort in the vehicle.
In accordance with the invention, the method of molding a tire having a tread provided with a sculpture, said sculpture having blocks of rubber surrounded by recesses formed at least in part by grooves, each of said blocks having a theoretical volume of rubber, said tread being molded by a segment mold, each segment being limited in circumferential direction by transverse faces, said segments being provided with elements in relief disposed in accordance with the arrangement desired in order to impress the recesses defining the sculpture of said tread, said elements in relief defining on the segments recessed zones the volume of which corresponds to said theoretical volume, some of such recessed zones being distributed over two adjacent segments, the method being characterized by the fact that those of said recessed zones which are distributed over two adjacent segments have a volume greater than the said theoretical volume of the corresponding block.
The invention also relates to segment molds designed in accordance with the conditions set forth. It can be applied both to molds, the transverse faces of the segments of which are radial, as is the most frequent case, and to molds the transverse faces of which are parallel to a sculpture motif, as well as to molds comprising furthermore a circumferential median separation defining two axially adjacent rings. Moreover, it extends to the tires produced by the method described.
In order to simplify the description of the invention, there will be described below its application for a given sculpture for heavy vehicle tires obtained by molding by means of a segment mold, the transverse faces of said segments being radial.
During the closing phase mold in a vulcanization press, it is desirable that the closing of the mold takes place before the commencement of the molding of the sculpture. In fact, most of the tread sculptures have furrows, whether linear or not. In order to assure the molding of these furrows, there are elements in relief which protrude radially towards the inside on the inner surface of the segments since the latter comprise what is commonly referred to as the negative of the sculpture to be molded. By the formation "in segments" of the molding ring of the tread, these elements in relief are interrupted at the level of the transverse faces of the segments, also referred to as "joint planes of the segments." Therefore, if one starts the molding before the segments are all together, that is to say, before the complete closing of the mold, there is the danger of imprisoning rubber between the segments of the molds, at least at the level of the elements in relief.
If, in order to avoid this problem, the tire is built in such a manner that in raw state the outer surface of the tread is located at a radial level just lower than the elements in relief of the mold when the latter is closed, then the additional shaping made necessary in order for the tire to reach its final dimensions causes sliding of the reinforcement cords (carcass plies and crown plies) and changes in angles which these cords form with respect to each other. Furthermore, this additional shaping can be made impossible in the case of filiform reinforcements arranged below the tread at 0.degree. with respect to a plane perpendicular to the axis of rotation.
Therefore, it is often the practice to dimension a tire in raw state in such a manner that the start of the penetration of the elements in relief of the segments of the mold into the rubber of the tread takes place before the mold is completely closed.
This gives rise to the appearance of a defect which had never been observed before in the case of sculptures having blocks of rubber surrounded on all sides by recesses. In the following description, these recesses will be called "grooves" if their width is greater than 2 mm and "incisions" if not. Frequently, these incisions are closed when the tire is inflated; they are closed at least in the area of contact with the road, while the grooves are intended, in particular, to remove water in rainy weather and must therefore remain open.
Whatever the type of recess, the molding problem arises when the sculpture comprises blocks surrounded on all sides by recesses. At the level of the joint planes between the segments, the recesses corresponding to the blocks of rubber are generally distributed over the two adjacent segments. This limits the appearance of the burrs referred to above and also simplifies the formation of the segments and the control of the movements of the vulcanization press.
It has been found that the vibrations complained of corresponded to a defect in circularity of the tire of harmonic H6 to H16, corresponding to the number of segments of the mold.
It has been found that a defect in geometrical regularity can even appear when the sculpture comprises blocks which are connected together by bridges of rubber, in particular when the sculpture comprises large circumferential furrows and transversely arranged incisions or grooves, without these incisions or grooves traversing the rib of rubber on both sides. Therefore, more generally, it is not necessary for the blocks to be surrounded on all sides by recesses; as soon as the elements of the mold assuring the molding of the sculpture constitute an impediment to the movements of still unvulcanized rubber in circumferential direction by complete blocking or at least by causing a loss in head sufficiently great to limit the flow, and therefore the mass transfer, geometrical defects appear having harmful results in operation.
As a further example, on certain tires for passenger cars, different amounts of wear have been found at the places of the tread corresponding to the joints between segments as compared with the places of the tread corresponding to the centers of the segments.